Add data manifests

[oliverfoster]

Subject of the enhancement

Adapt data structures are confined to JSON files for config, course, contentObjects, articles, block and components.
Add a data file manifest to allow reorganisation and/or extension of data files.

1. grunt to publish manifest of course/*/*.<%= jsonext %> files at course/*/language_data_manifest.js
2. framework to load course/*/language_data_manifest.js

As the JSON is more fluid, we'll need code abstractions for Languages and Schemas. This will be to facilitate:

• translations (json<>languagefile) issue/2662 Formalised plugin, data, schema and translate API #2667
• course._globals defaults to be populated (schema>json) issue/2662 Formalised plugin, data, schema and translate API #2667
• schema inheritance and blending Formalise plugin schemas #2704